In regard to critizism of fans not hiding their disapproval of players (performances)...one could maybe also see the fans voice as a Kind of `audience joker` a coach could take into account. In my eyes coaches are influenced by the fans reactions towards players. A good Coach would take such approval/disapproval into account. Of course he would have to be able to understand what the motivation behind such reactions are. Are they really appropritate in regard to a players performances or are the motivs others.
I`m definately not saying that the most and loudest voices are those who should dictate a coaches choice of players. But more often than not, it seems that the fans see things in a more appropiate way than a coach does, if they ask for a player to be played or the other way round.
For a coach the `audience` can be an important source of feed-back, but a rather problematic one at the same time.
In regard to the players themselves the fans reactions towards their performances (good or bad) in an appropiate way is simply a feed-back they should recognize without under- or overestimating such. Ideally any reaction should serve as an incentive and indicator to become a better player. (As fans will let their feelings known, more so if results aren`t as they hope for, it should be something every professional player should learn to cope with. As should every coach of course if the critizism is directed at his person.)

In an ideal world fans would show their feelings, good or bad, towards players/coach/owners in a way that suits the critizised players mentality. Some react positive to a `kick up the backside` some don`t.
The ideal fan would be a psychologist treating every player in a way enabling him to react in a best possible way. Some players need `a slap in the face`. Others react more positive to `tender loving care`. The latinist would choose the phrase `quique suum`...